For example, titanium-containing oxides having a perovskite crystal structure or a layered perovskite crystal structure, which are represented by MTiO3 (M represents Ca, Ba, Mg, Sr, or Pb) and titanium-containing oxides in which some of Ti atoms are substituted with at least one type of Hf and Zr (PTL 1) (collectively referred to as “titanium-containing perovskite oxides”) have a considerably high relative dielectric constant. Therefore, the titanium-containing perovskite oxides have been eagerly studied for a long time as devices such as capacitor materials and dielectric films and also in terms of applications to substrate materials of other perovskite transition metal oxides and nonlinear resistors.
In addition to the excellent characteristics, the fact that titanium is an element which has a low environmental load, is safe for living bodies, and is abundant on the earth facilitates the use of titanium-containing perovskite oxides for biocompatible materials and the industrial use of titanium-containing perovskite oxides for electronic materials, optical materials, and the like. Among Clarke numbers, which represent the proportions of elements present in the earth's crust, titanium is in tenth place among all elements and in second place after iron among transition metals.
It is known that titanium compounds in which the valences of Ti are +4 (3d0) and +3 (3d1) are stably present. The material development of titanium-containing oxides that uses the conductivity of such d electrons has been eagerly conducted. For example, Nb-doped anatase (TiO2) is promising as a transparent electrode material, and Ti4O7 known as one of Magneli phases is promising as a switching material (NPL 1) because Ti4O7 exhibits a metal-insulator transition.
It is known that, by forming oxygen defects (vacancies) in an insulative titanium-containing perovskite oxide, that is, by doping the insulative titanium-containing perovskite oxide with electrons, a mixed valence state with titanium having valences of +3 and +4 is achieved and thus the insulative titanium-containing perovskite oxide can be converted into a material with low electrical resistance (NPL 2). To achieve this, various methods such as a heat treatment at high temperature in vacuum, in hydrogen, in nitrogen, or in argon gas, using an oxygen getter are employed.
Regarding oxides, an increasing number of studies have been conducted on, for example, an oxide-ion mixed conductor (PTL 2) and an electrochemical device including a material having proton (H+) ion conductivity as a solid electrolyte (PTL 3). On the other hand, almost no studies have been conducted on negatively charged hydride ions (H−). The possibility that hydrogen in an oxide is conducted in the form of hydride ions was proposed by S. Steinsvik et al. in 2001 (NPL 3). However, there are opposing opinions on this theory and its validity is still disputed.
In general, the compatibility between oxide ions and hydride ions is very poor. Therefore, successful examples in which hydride ions are inserted into an oxide in an amount exceeding the amount of oxygen defects are limited to only a small number of substances that use typical elements.
Examples of such substances include LaHO (NPL 4) and 12CaO.7Al2O3 (NPL 5 and PTL 4).
In 2002, M. A. Hayward et al. succeeded in synthesizing a cobalt oxide-hydride containing hydride ions, LaSrCoO3H0.7 (NPL 6). After that, in 2006, C. A. Bridges et al. reported the diffusion phenomenon of hydride ions in the cobalt oxide LaSrCoO3H0.7 (NPL 7). This indicates that hydride ions in the substance have mobility, but does not indicate a chemical reaction with the ambient atmosphere (e.g., gaseous phase). The ion conductivity is unknown. Furthermore, R. M. Helps et al. reported a cobalt oxide-hydride having a structure similar to that of the substance, Sr3Co2O4.33H0.84 (NPL 8). These two substances are the first examples in which a large amount of hydride ions were taken into a transition metal oxide.    PTL 1: Japanese Unexamined Patent Application Publication No. 2006-199578    PTL 2: Japanese Patent No. 4374631    PTL 3: Japanese Unexamined Patent Application Publication No. 2005-100978    PTL 4: Japanese Patent No. 4219821    NPL 1: S. Ohkoshi et al., “Nature Chemistry” 2, p. 539-545 (2010)    NPL 2: W. Gong et al., “Journal of Solid State Chemistry” 90, p. 320-330 (1991)    NPL 3: S. Steinsvik et al., “Solid State Ionics” 143, p. 103-116 (2001)    NPL 4: K. Hayashi et al., “Nature” 419, p. 462-465 (2002)    NPL 5: B. Malaman, J. F. Brice, Journal of Solid State Chemistry” 53, p. 44-54 (1984)    NPL 6: M. A. Hayward et al., “Science” 295, p. 1882-1884 (2002)    NPL 7: C. A. Bridges et al., “Advanced Materials” 18, p. 3304-3308 (2006)    NPL 8: R. M. Helps et al., “Inorganic Chemistry” 49, p. 11062-11068 (2010)